The present invention relates generally to methodology and apparatus for treatment to sleep apnea and, more particularly, to mono-level, bi-level, or proportional assist ventilation (PAV) continuous positive airway pressure (CPAP) apparatus including circuitry for enabling a patient to selectively actuate one or more pressure ramp cycles wherein, during each ramp cycle, available airway pressure increases with time from a predetermined minimum pressure value to a prescription pressure, thereby facilitating the patient""s transition from a waking to a sleeping state.
The sleep apnea syndrome afflicts an estimated 1% to 3% of the general population and is due to episodic upper airway obstruction during sleep. Those afflicted with sleep apnea experience sleep fragmentation and intermittent, complete or nearly complete cessation of ventilation during sleep with potentially severe degrees of oxyhemoglobin unsaturation. These features may be translated clinically into debilitating daytime sleepiness, cardiac dysrhythmias, pulmonary-artery hypertension, congestive heart failure and cognitive dysfunction. Other sequelae of sleep apnea include right ventricular dysfunction with cor pulmonale, carbon dioxide retention during wakefulness as well as during sleep, and continuous reduced arterial oxygen tension. Hypersomnolent sleep apnea patients may be at risk for excessive mortality from these factors as well as by an elevated risk for accidents while driving and/or operating potentially dangerous equipment.
Although details of the pathogenesis of upper airway obstruction in sleep apnea patients have not been fully defined, it is generally accepted that the mechanism includes either anatomic or functional abnormalities of the upper airway which result in increased air flow resistance. Such abnormalities may include narrowing of the upper airway due to suction forces evolved during inspiration, the effect of gravity pulling the tongue back to appose the pharyngeal wall, and/or insufficient muscle tone in the upper airway dilator muscles. It has also been hypothesized that a mechanism responsible for the known association between obesity and sleep apnea is excessive soft tissue in the anterior and lateral neck which applies sufficient pressure on internal structures to narrow the airway.
The treatment of sleep apnea has included such surgical interventions as uvulopalatopharyngoplasty, gastric surgery for obesity, and maxillo-facial reconstruction. Another mode of surgical intervention used in the treatment of sleep apnea is tracheostomy. These treatments constitute major undertakings with considerable risk of postoperative morbidity if not mortality. Pharmacologic therapy has in general been disappointing, especially in patients with more than mild sleep apnea. In addition, side effects from the pharmacologic agents that have been used are frequent. Thus, medical practitioners continue to seek non-invasive modes of treatment for sleep apnea with high success rates and high patient compliance including, for example in cases relating to obesity, weight loss through a regimen of exercise and regulated diet.
Recent work in the treatment of sleep apnea has included the use of continuous positive airway pressure (CPAP) to maintain the airway of the patient in a continuously open state during sleep. For example, U.S. Pat. No. 4,655,213 and Australian patent AU-B-83901/82 both disclose sleep apnea treatments based on continuous positive airway pressure applied within the airway of the patient.
Also of interest is U.S. Pat. No. 4,773,411 which discloses a method and apparatus for ventilatory treatment characterized as airway pressure release ventilation and which provides a substantially constant elevated airway pressure with periodic short term reductions of the elevated airway pressure to a pressure magnitude no less than ambient atmospheric pressure.
Published PCT Application No. WO 88/10108 describes a CPAP apparatus which includes a feedback system for controlling the output pressure of a variable pressure air source whereby output pressure from the air source is increased in response to detection of sound indicative of snoring. A pressure ramp cycle (i.e., a gradual increase in output pressure) occurs upon initial activation of the apparatus while other ramp cycles occur automatically thereafter upon detection of snoring sounds from the patient.
Publications pertaining to the application of CPAP in treatment of sleep apnea include the following:
1. Lindsay, D A, Issa F G, and Sullivan C. E. xe2x80x9cMechanisms of Sleep Desaturation in Chronic Airflow Limitation Studied with Nasal Continuous Positive Airway Pressure (CPAP), xe2x80x9cAm Rev Respir Dis, 1982; 125: p. 112.
2. Sanders N H, Moore S E, Eveslage J. xe2x80x9cCPAP via nasal mask: A treatment for occlusive sleep apnea, Chest, 1983; 83: pp. 144-145.
3. Sullivan C E, Berthon-Jones M. Issa F G. xe2x80x9cRemission severe obesity-hypoventilation syndrome after short-term treatment during sleep with continuous positive airway pressure, Am Rev Respir Dis, 1983; 128: pp. 177-181.
4. Sullivan C E, Issa F G, Berthon-Jones M., Eveslage J. xe2x80x9cReversal of obstructive sleep apnea by continuous positive airway pressure applied through the nares, Lancet, 1981; 1: pp. 862-865.
5. Sullivan C E, Berthon-Jones M. Issa F G. xe2x80x9cTreatment of obstructive apnea with continuous positive airway pressure applied through the nose. Am Rev Respir Dis, 1982; 125: p. 107. Annual Meeting Abstracts.
6. Rapoport D M, Sorkin B, Garay S M, Goldring R N. xe2x80x9cReversal of the xe2x80x98Pickwickian Syndromexe2x80x99 by long-term use of nocturnal nasal-airway pressure,xe2x80x9d N Engl J. Med, 1982; 307: pp. 931-933.
7. Sanders M H, Holzer B C, Pennock B E. xe2x80x9cThe effect of nasal CPAP on various sleep apnea patterns, Chest, 1983; 84: p. 336. Presented at the Annual Meeting of the American College of Chest Physicians, Chicago Ill., October 1983.
8. Sanders, M H. xe2x80x9cNasal CPAP Effect on Patterns of Sleep Apneaxe2x80x9d, Chest, 1984; 86: 839-844.
Although CPAP has been found to be very effective and well accepted, it suffers from some of the same limitations, although to a lesser degree, as do the surgery options; specifically a significant proportion of sleep apnea patients do not tolerate CPAP well. Thus, development of other viable non-invasive therapies has been a continuing objective in the art.
The present invention contemplates a novel and improved method for treatment of sleep apnea as well as novel methodology and apparatus for carrying out such improve treatment method. The invention contemplates the treatment of sleep apnea through application of pressure at variance with ambient atmospheric pressure within the upper airway of the patient in a manner to promote dilation of the airway to thereby relieve upper airway occlusion during sleep.
In a first embodiment of the invention, positive pressure is applied at a substantially constant pressure within the airway of the patient to maintain the requisite dilating force to sustain respiration during sleep periods. This form of treatment is commonly known as mono-level CPAP therapy.
In another embodiment of the invention, positive pressure is applied alternately at relatively higher and lower pressure levels within the airway of the patient so that the pressure-induced force applied to dilate the patients airway is alternately a larger and a smaller magnitude dilating force. The higher and lower magnitude positive pressures are initiated by spontaneous patient respiration with the higher magnitude pressure being applied during inspiration and the lower magnitude pressure being applied during expiration. This method of treatment may descriptively be referred to as bi-level CPAP therapy.
The invention further contemplates a novel and improved apparatus which is operable in accordance with a novel and improved method to provide sleep apnea treatment. More specifically, a flow generator and an adjustable pressure controller supply air flow at a predetermined, adjustable pressure to the airway of a patient through a flow transducer. The flow transducer generates an output signal which is then conditioned to provide a signal proportional to the instantaneous flow rate of air to the patient. The instantaneous flow rate signal is fed to a low pass filter which passes only a signal indicative of the average flow rate over time. The average flow rate signal typically would be expected to be a value representing a positive flow as the system is likely to have at least minimal leakage from the patient circuit (e.g., small leaks about the perimeter of the respiration mask worn by the patient). The average flow signal is indicative of such leakage because the summation of all other components of flow over time must be essentially zero since inspiration flow must equal expiration flow volume over time, that is, over a period of time the volume of air breathed in equals the volume of the gases breathed out.
Both the instantaneous flow signal and the average flow rate signal are fed to an inspiration/expiration decision module which is, in its simplest form, comparator that continually compares the input signals and provides a corresponding drive signal to the pressure controller. In general, when the instantaneous flow exceeds average flow, the patient is inhaling and the drive signal supplied to the pressure controller sets the pressure controller to deliver air, at a preselected elevated pressure, to the airway of the patient. Similarly, when the instantaneous flow rate is less than the average flow rate, the patient is exhaling and the decision circuitry thus provides a drive signal to set the ressure controller to provide a relatively lower magnitude of pressure in the airway of the patient. The patient""s airway thus is maintain e d open by alternating higher and lower magnitudes of pressure which are applied during spontaneous inhalation and exhalation, respectively.
As has been noted, some sleep apnea patients do not tolerate standard CPAP therapy. Specifically, approximately 25% of patients cannot tolerate CPAP due to the attendant discomfort. CPAP mandates equal pressures during both inhalation and exhalation. The elevated pressure during both phases of breathing may create difficulty in exhaling and the sensation of an inflated chest. However, we have determined that although both inspiratory and expiratory air flow resistances in the airway are elevated during sleep preceding the onset of apnea, the airway flow resistance may be less during expiration than during inspiration. Thus it follows that the bi-level CPAP therapy of our invention as characterized above may be sufficient to maintain pharyngeal patency during expiration even though the pressure applied during expiration is not as high as that needed to maintain pharyngeal patency during inspiration. In addition, some patients may have increased upper airway resistance primarily during inspiration with resulting adverse physiologic consequences. Thus, our invention also contemplates applying elevated pressure only during inhalation thus eliminating the need for global (inhalation and exhalation) increases in airway pressure. The relatively lower pressure applied during expiration may in some cases approach or equal ambient pressure. The lower pressure applied in the airway during expiration enhances patient tolerance by alleviating some of the uncomfortable sensations normally associated with CPAP.
Under prior CPAP therapy, pressures as high as 15 cm H2O have been required, and some patients on nasal CPAP thus have been needlessly exposed to unnecessarily high expiratory pressures with the attendant discomfort and elevated mean airway pressure, and theoretic risk of barotrauma. Our invention permits independent application of a higher inspiratory airway pressure in conjunction with a lower expiratory airway pressure in order to provide a therapy which is better tolerated by the 25% of the patient population which does net tolerate CPAP therapy, and which may be safer in the other 75% of the patient population.
As has been noted hereinabove, the switch between higher and lower pressure magnitudes can be controlled by spontaneous patient respiration, and the patient thus is able to independently govern respiration rate and volume. As has been also noted, the invention contemplates automatic compensation for system leakage whereby nasal mask fit and air flow system integrity are of less consequence than in the prior art. In addition to the benefit of automatic leak compensation, other important benefits of the invention include lower mean airway pressures for the patient and enhanced safety, comfort and tolerance.
In all embodiments, the present invention makes use of xe2x80x9crampxe2x80x9d circuitry operatively connected to pressure control means of the CPAP apparatus and selectively activatable by the patient to effect at least one pressure xe2x80x9cramp cyclexe2x80x9d which is described in greater detail below. The maximum duration of the ramp cycle, the shape of the ramp curve and the prescription pressure are normally established by a sleep study of the patient and this data can be programmed into the CPAP apparatus of the instant invention. It is also desirable that the CPAP apparatus be operable either by manual controls located directly on the apparatus or via remote control.
Approximately 25% of all patients who undergo CPAP therapy for sleep apnea experience respiration discomfort and find it difficult to fall asleep because of the therapy. The purpose of a ramp cycle is to alleviate this discomfort. A ramp cycle is an automatic cycle that, once activated, causes the CPAP apparatus to output a predetermined minimum positive pressure at or above ambient pressure which is gradually increased over a predetermined time period known as xe2x80x9cramp timexe2x80x9d during which the patient begins to fall asleep. Upon expiration of the ramp time the patient typically has fallen asleep and at such time the pressure produced by the apparatus is that of the patient""s CPAP therapy prescription pressure whereupon the patient receives normal CPAP treatment as he sleeps.
A particular advantage of the present invention is that the unique ramp circuitry enables not only an initial ramp cycle to be achieved for when one first attempts to sleep but such circuitry also permits one or more additional cycles to be selectively activated by the user at instances where the user awakens during an extended rest period and again requires a ramp cycle to fall back to sleep. Typically, during a sleeping period of several hours, the time required to once again fall asleep after briefly being awakened is generally less than the time spent initially falling asleep. To accommodate this phenomenon, the ramp circuitry of the instant invention allows the user to advantageously adjust the ramp time of any additional ramp cycle to run for a selected fraction of the initial ramp time, which itself is a patient-selected fraction of a prescription pressure preset by a health care professional in supervision of the patient""s sleep apnea treatment.
The ramp circuitry enables a physician or other health care worker to set the initial ramp time and prescription pressure. Additionally, however, the novel ramp circuitry of the present invention permits adjustment of the xe2x80x9cshapexe2x80x9d of the pressure ramp curve, whereby the physician, health care worker or patient can suitably manipulate appropriate controls associated with the ramp circuitry to control the pressure output pattern of the ramp (as represented as a function of pressure versus time) such that it may assume virtually any configuration including, inter alia, linear, stepped, or curvilinear slope, depending upon a patient""s particular needs as dictated by the results of the patient""s sleep study.
Additionally, sufferers of sleep apnea are sometimes afflicted by other maladies which limit the degree to which they may safely physically exert themselves. An advantage of the present invention is that it enables a limited-mobility user, at his discretion, to operate the CPAP apparatus either by manual controls located directly on the apparatus or via remote control. Equally as important, it provides any sleep apnea sufferer using the CPAP apparatus with the peace of mind of knowing that the pressure can be reduced at any time via the remote control. Further, the preferred embodiment of the remote control contemplated for use in the present invention is one which the user can operate easily and reliably either in light or darkness to turn the apparatus on and off as well as selectively activate the first or subsequent ramp cycles.
Further, although the ramp circuitry discussed hereinbelow will be described specifically in connection with mono-level and bi-level CPAP apparatus, it will be understood that its utility and applicability is not limited thereto. That is to say, within the scope of the instant invention it is also contemplated that the presently disclosed ramp circuitry may be incorporated into other types of CPAP apparatus including, but not limited to, proportional assist ventilation (PAV) devices which are similar to bi-level CPAP devices but instead provide substantially continuous adjustment of pressure in response to patent volume and flow instead of alternating between two fixed pressures in response to flow.
Other details, objects and advantages of the present invention will become apparent as the following description of the presently preferred embodiments and presently preferred methods of practicing the invention proceeds.